Apparatus for reducing crosstalk currents



May 24, 1932.

c. R. KEITH l APPARATUS FOR REDUCING' CROSSTALK CURRENTS lFiliad Dec. 4, 1928 2 sheets-sheet 1 YN .QQ

.w l mi n.. n. N.. a. m. or o a siv e ou be( /vou wund/w 39u70 /N VE N TUR E. Km TH May 24, 1932. c. R. KEITH 1,859,555l -APPARATUS( FOR REDUCING CROSS'VIALK CURRENTS Filed vec. 4. 192e 2 sheets-sheet 2 a v qu 'u l 'se -n b. E eg -N s .mi 2S M" .2

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O O O /NVf/V TOR Patented May 2 4, 1932 YRK onANGE, NEW Jansma INCORPORATED, or NEW Yonx,

AssrGNon 'ro BELL mmnoNn N. Y., a conromrIoN or maw APPAIRAEIS REDUQING CROSSTALK U B.B.IIQN"JSl Application led December 4, 1928. Serial No. 328,724.

T'his invention relates to transmission systems and particularly to circuits nating between currents of different amplitudes. c

It is well known that interfering sounds are more noticeable ring alone than when accompanied by sounds of much greater intensity, since in the latter instance they are masked out to a considerable extent. On thig account noise or crosstalk currents are more objectionable during pauses in conversation than when accompanied by speech.

The maximum allowable amount of crosstalk is governed by the amount permissible during the pauses in conversation since this amount is less than that permissible during the talking periods. In other words a very much smaller amount of crosstalk will become understandable -or objectionable during silent intervals than that which would be required to detrimentally a'ect the intelligibility of speech during talking periods.

An object of this invention is to render a 5 circuit capable of discriminating between currents of different amplitudes. v

Another object of the invention is to cohtrol the amplification of an amplifying de'- vice in accordance with signals transmitted over the line.

A further object of the invention is'to vary the conductivity of a vacuum tube translating l device in a manner determined by the current transmitted therethrough.

In order to accomplish these objects, 'a circuit is equipped with an amplifier or someother suitable arrangement of space discharge devices and means are provided for causing.

the transmission eiiiciency of the circuit to vary so that the system has considerably less gam 1n response to small currents than in ret sponse to signal currents.

In one aspect of theinvention an amplifier is connected in a transmission line having a portion of its voutput current rectined and fed back through a'resistance in the grid circuit of the amplifier causing-a voltage drop thereacross in opposition to the normal negative grid voltage on `the tube. When the current on the line is small as `for instance for discrimiand objectionable occur- -of' speech the current available .control electrode.

"cuit of the amplifier that resulting from crosstalk during pauses in conversation, thev amount of tified is negligible and the normally hi h negative voltage on its 'grid' effectively bl the transmission of current through the amplifier. However, during the transmission in the output of the amplifier for rectification andapplication upon the amplifier input circuit is considerably larger and is suicient to reduce the eects ofthe negative grid bias on the am lifer grid'electrode, so that' normal am'- pli cation of the signals is permitted.

In another embodiment of the invention Y the gain of a demodulator system is made de-- pendent upon the strength of signals-in its output circuit which 1s considerably larger for normal talking currents than for small curents corresponding to crosstalk.

n vention a similar but not effect is obtained by rectlying a portion of the in ut current before amplification by the ampli er and applying thereof through a resistance to produce a voltage in opposition to a high negative grid voltage normally impressed upon the amplilier. Asl in the other vcase the smaller crosstalk currents are prevented from being transmitted through the amplifier whereas signals of speech amplitude are 'amplified to a normal extent. Y

l A better understanding of the invention may be had` by reading the following descr1ption together drawings of which Fig. 1 show s an amplitude with the accompanyingl current recit to the grid circuit 'discriminator comprising an amplifier, the' output circuit of which supplies the current for controlling the biasing voltage on the Fig. 2 is a similar aronly in the of rectifier circuit used. ,Figs 3 illustrates the 'invention applied to a demodulator circuit. In Fig. 4 an amplifier similar to that of Fig. 1

is shown differing only in that current is rangement differing supplied to the rectifier fromtlie input cirinstead `of the output circuit to obtain the Vgrid bias.' Fig. -5 is similar to Fig. 4 except that 'the amplifier 1s the balanced or push-pull type and the .Inh

2, the battery 6 for impressing a negative grid fixed staticgrid bias is positive. Figs. l--A to 5-A are curvesillustrating respectively the operations of circuits 1 to 5.

VReferring now to Fig. 1 the amplitude discriminator comprises a vacuum tube' amplifier `1 connected tothe line L by means of a transformer 2. The ouput of the amplifier is'connected to a lin'e or any suitable receiving apparatus through the transformerV 3. The amplifier is of the three element space discharge type comprising ,a plate maintained at a suitable potentialby means of a source of energy 11, a filament suitably/ener'- gized from a source of filament current 1:2. and a grid connected to the filament through the secondary winding 13 of the transformer potential upon the grid and the resistance 7. Resistance 7 is shunted by a condenser 8, the purpose of which will be hereafter described.l Shunted across the output circuit of the v amplifier is the primary 14 of the transformer 4, to the secondary winding 15 of which is connected the rectifier 5 for supplying rectified current to the resistance 7, to producetherein a voltage which tends to reduce the negative bias on the grid of the amplifier 1. I

' The input circuit of the rectifier may be traced from the grid 19, through the resistance 16, transformer winding 15, negative 'grid battery 17 tothe filament 10. The output circuit of the rectifier 5 may be traced from filament 10 through the resistance 7,

battery 11, the lead 18 to the plate 9.

The operation of the circuit may be readily understood by reference to the vcurve of Fig.`

l1---A which shows A. C. input voltage to the amplifier as abscissa plotted against fthe amplification ratio of input to output voltage. ySmall voltages corresponding to crosstalk on the line are substantially suppressed as shown by portion' a of. the curve. This results from the fact -that current which passes through the resistancel 7 from the rectifier is so small as tohave practically no effect upon the fixed negative grid potential produced by source 6, which, under the'se4 circumstances, transmission through` the tube '1. As the voltage on the wire is increased,l however. a

critical value b is reached above which a sufiicient amount of current is amplified by the tube 1, rectified by tube 5, and passed 'through resistance 7 to produce a voltage in opposition to the voltage of battery 6. .This reduces the C bias on-the' grid of amplifier tubel sufiiciently to permit it to`transmit current with a relatively high gain as shown by portion c ofthe curve .in Fig.'1-A. A lIn passing fromthe region of low gain, a, to the region of high gain, c, a 'region of instability b exists due to regeneration as the optimum value of grid potential is being built .up. This has no effect upon the transmission is sufcientlylarge to prevent of signals, however,

as the signal voltages u are normally considerably greater than the y voltages in the unstable region. As the voltnomenon is advantageous', however, since it tends to keep the transmission level of the system constant between words and sentences of the conversation.

The condenser 8 which shunts the resistance'7 acts as a filter to prevent alternating currents transmitted by the rectifier 5 from being impressed on the grid of amplifier 1. This prevents singing through the amplifier and rectifier circuits. At the-same time condenserV 8 provides a. low impedance to -unwanted frequencies inthe output of rectifier 5 and so allows the rectifier to operateat maximum efficiency. The condenser also tends to keep the gain'of the amplifier from varying so rapidly that the speech or signals sound choppy. In order to make the change of gain with input voltage as large as possible after the maximum crosstalk voltage has been reached, the rectifier should have its maximum gain for voltages just above this value and lower gain voltages. The lower gainfor higher voltages is quite important since otherwise .the effec- C bias of the amplifierwould decrease t0 zero for large input voltages resulting in a decrease 'of both gain andquality.

The rectifier current output may be made to rise rapidly to a maximum and then stay practically. constant by means ofthe resistance 16 in the grid circuit of therectifier tube. Withthe rectifier grid battery 17 .of

such a value that the initial space current' both for lower voltages and higher los very little and consequently the rectified current in resistance 7 remains substantially constant. By selecting the proper grid biasing resistance 7 for the am lifier tube 1 the effective biasfor maximum input voltage can be made very close to the optimum.

In an actual vcircuit as used by applicant,

which resulted in the experimental curve in Fig. 1-A, the amplifier tube land rectifier tube 5 were standard Western^Electric tubes designated respectively 101-D and 102-D.

The transformers'were of the voice frequency type, the input-.transformer 2 having an impedance ratio of 600 to 100,000 ohms, the output transformer 6000 to' 600 ohms and the transformer 4 connecting the rectifier' with I the amplifier 18,000 to 300,000 ohms.

The resistance 16 whichcontrols the ran e over which the amplifier can operate wit duced to approximately one hundredth of this' amplitud Thus, with the value of circuit elements given above, the normal speech voltage working into a 600 ohm resistance averages .3 voltsfwhile crosstalk up to .03 volts is suppressed.

Another embodiment of the invention is shown in Figs. 2 and 2-A. Since this is similar to Fig. 1 in many respectscorre sponding elements have been given the samev designation. This circuit operates 1n a many ner similar to that shown in Fig. 1, the main difference residing in the type of rectifier which supplies current to the resistance 7 in the amplifier grid circuit. In this circuit a portion of the current from the output of the amplifier 1 is impressed upon the input of the v vacuum tube amplifier 20 through the transformer 21. The amplified current from the output of the amplifier 20 is impressed through transformer 23 upon the group of rectifiers 22 which are arranged to rectify current to both polar-ities.v Thus when current in the secondary of the transformer 23 1s in the direction of the arrow, rectified curv rent through the resistance'? may be traced from the transformer secondary through the rectifier 24, resistance 7, rectifier 27 and back to the lower terminal of the transformer winding 28. When the current in winding 28 is in theopposite direction its path lies through the rectifier 25, resistance 7 and rectifier 26 to tlie upper end of the transformer winding 28. One advantage of -this circuit resides in the fact that onlya single source of filament heating current is required since the rectifiers of this circuit may be of the crystal or copper oxide type which require no filament battery. However, the circuit is not limited to this type of rectifier since any type of rectifier may be successfully used. In order to simplify the drawings, two filament batteries have been shown, although the manner in which a single battery may be used lfor both amplifiers will be at once apparent to those skilled in the art. The opera- Ation 'of this'circuit is very similar to that of the circuit of Fig. 1 and, therefore, will not be described. The operating characteristie of this circuit is shown in Fig. 2A-A.

. acteristic curve of Fig. 1-A. p v

' The `circuits disclosed inI the foregolng' fig' which is similar in most respects to the charl ures and .described above are suitable for use in systems employing speech frequencies'on the transmission line. Fig. 3 discloses a type noise or Crosstalk suppression circuit which may be used in connection with radio or carrier wave transmission Since vthe gain of a plate circuit modulator or demodulator varies with the grid bias in a manner similar to an amplifier, crosstalk reduction may also be effected in accordance with the invention in this type of'v circuit. A circuit accomplishingthis result is shown in Fig. 3. A demodulator of welly known type is shown comprising a pair of three-element vacuum tubes 30 and 31 in balanced relation, the grid electrodes of which are connected together through the vbranches 35 and 36 of the secondary winding of the transformer 34. The mid-point of the transformer is connected to the filaments of the tubes through the secondary winding 37 of the transformer 38, the resistance 39 and the battery 41 which are in the common branch of the demodulator input circuit. The band pass filter 42 for excluding all but the desired frequency band is connected to the prilmary of the transformer 34 so that a freprovides an optimum impedance for the rectifier 22. Rectified current from the rectifier amplifier flows through the resistance 7 producing"a.voltage drop in opposition to the negatlve grid-potential of battery 41. The rectified current is dependent upon the current in the demodulator output since the input ofthe rectifier amplifier is connected to the output of the demodulator through the transformer 21. r

The rectifier amplifier is similar to the one shownin Fig. 2 and therefore will not be described in detail. The operation of the circuit with reference to the varying grid potential for the reduction of crosstalk is similar to that of the circuits of Fig. land Fig. y2. The ratio of side band output voltage Y to signal input voltage of the circuit for vainput circuit of vacuum tube amplifierv 53` point of the transformer secondary winding to speech.

' amplifier comprising vacuum tubes 60 b, c and d of the curve correspond generally to the corresponding portions olf the curves vfor an amplifier.

Fig. 4 illustrates a crosstalk reducing circuit comprising an amplifier with a variable grid bias similar to that shown in Fig. 1. However, in this circuit the current to be rectified is obtained from the input of the arnplifier, instead of from the amplifier output, as in the circuits heretofore described. Referring to Fig. 4 the transformer 50 has its primary winding51 shunting the line L and its secondary winding 52 connectedto the which is of any well known type. The output of the amplifier is connected to the input circuit of the rectifier, 54 through the transformer 55. The resistance 56 and negative grid battery 57 in the input circuit are adjusted to a value which 'permits maximum gain for voltages above the maximum crosstalk lvoltage and lower gain for voltages above and below this value. The latter is essential since should the grid impedance of the amplifier be made too small, most of the voltage drop in the amplifier input circuit would be across the transformer -winding resulting in very inefficient transmission through the amplifier. The output circuit of the rectifier traced from the filament 58 through the resistance 7 is poled to reduce the grid biasing voltage 6.,y As in the circuits already described, small voltages corresponding to crosstalk have practically no effect on the negative grid biasing voltage which is large enough to prevent transmission through the amplifier. However, the signal voltages are considerably larger and reduce the negative grid voltage sufficiently to prevent: the blocking action and permit normal transmission through the amplifier 1.

Fig. 4-A shows the operating characteristic of this circuit, as experimentally obtained. It is seen from the curve that the crosstalk is effectively discriminated against, relative Suppression' of small voltages in a variable bias amplifier may also be-obtained by using an initially positivebias. A' circuit of this type is shown in Fig. 5. Herein a push-pul an 61 is connected to the line L through transformer 62. The grids of the amplifier are connected together through the upper and lower branches of the secondary winding. 63 of the transformer L62. The grid circuits of the vacuum tubes are completedl through the common branch which connects the mid- 63 to the filaments, and comprises resistance 64 and a source of positive4 grid potential 65. The output of the amplifier is arranged in accordance with the well 'known push-pull circuit.

An amplifier rectifier similar to that in to said amplifier for amplification Fig. 4 is connected to the input of the ushpull amplifier. The output circuit o the rectifier may be traced from the filament 58, to the filaments of vacuum tubes and 61, the source of potential 65, resistance 64, back to the plate 59 of the rectifier through the source of potential 66. In this circuit the voltage drop through thejresistance 64 is poled to decrease grids of the amplifiers 60 and 61 so that as the voltage in the input circuit L increases the variable grid bias is decreased. AAccording to the theory of operation of this circuit, small voltages upon the line, have little effect on high posltive grld-bias which makes the grid impedance of Athe amplifiers very low. Under vthese conditions most of the voltage drop in the grid circuits is across the `transformerwinding and very little is impressed upon the grid to be repeated tothe output circuit. However, larger voltages corresponding to speech on the line increase the amount of current rectified and reduce the positive bias on the amplifiers 60 and 61 to a point where the impedance of the transformer` matches the grid impedance `ofthe tube under whichcircumstances maximum transfer of energy results. The operation of this circuit is illustrated by the curve,`in Fig. 5-.A,- which shows negligible transmission for currents of small crosstalk amplitudes and good transmission for currents corresponding to signal amplitudes.

It will be apparent that the circuits described herein are typical of many which will suggest themselves to 4t-hose skilled in the the Ypositive bias` on the art within the spi-ritof the invention. The

invention is to be limited therefore only by the scope of the appended claims.

lWhat is claimed is: f

1. In a signal transmission system for discriminating between currents of different amplitudes, a space discharge amplifier for amplifying waves of normal signal amplitude f substantiall without distortion, a source ofA signals or supplying signal waves therein, said amplifier having .a control electrode associated with said source, a source of biasing potential for said electrode for preventing substantial amplification of currents small in comparison with said signal currents and means for varying said biasing potential in accordance with the amplified current yfrom said device to make itl readily amplif currents of' normal signal magnitude su stantially without distortion.

2. The combination in a transmission system of a line forthe transmission of signals, and means for the suppression of disturbing currents therein comprising an amplifier therein for amplifying substantially with out distortion waves of normal signal amplitude impressed on said amplifier from said line, said amplifier being normally opaque to currents of crosstalk amplitude but having means dependent upon the current transmitted through said amplifier for rendering the gain thereof as a distortionless am liier high for` currents of normal signal amplitude.

3. The combination in a transmission system of a line for the transmission of signals, means for the suppression of disturbing'currents therein comprising an amplifier therein for amplifying substantially without distor-v tion waves of normal signal amplitude impressed on said amplifier from said line, said amplifier being normally opaque to cur-rents of-crosstalk amplitude, and means for rectiing a portion of the energy transmitted t rough said amplifier and utilizing it for rendering the gain of said amplifieras a distortionless amplifier high for currents of normal signal amplitude.` 4. The combination in a transmission system of a-line for the transmission of signals and means for the suppression of disturbing currents therein comprising a space discharge amplifier'device therein for amplifying substantially without distortion waves of normal signal amplitude impressed on said device from said line, said device having a 'control element, means for impressing a voltage upon said control element of value suicient to prevent substantialamplification in said device of currents of crosstalk amplitude, and means for reducing the eect of said voltage in response to increase of the currents transmitted through said device to permit readily the amplification of currents of normal signal amplitude substantially without distortion.

5. The combination in a transmission system of aline for the transmission of signals.

and means for the suppression of disturbing currents therein comprising a space discharge amplifier device therein for amplifyin substantially without distortion waves o normal signal am litude impressed on said device from said) line, said device having an input circuit and an 'output circuit, means comprising a source of potential in said input circuit for rendering saiddevice opaque to disturbing currents small in com arison with currents of normal signal amplitude, a

' resistance in said input -clrcuit,'means for rectifying a 'portion of current from said output circuit and transmitting it through said resistance to produce a vpotential lto oppose said other source of potential rendering said device readily ing substantially without distortion currents of normal signal amplitude.

6. The combination in a transmission system of a line for the transmission of signals and means for the suppression of disturbingcurrents therein comprising a space discharge amplifier device therein for amplifying substantially without distortion waves of normal signal amplitude impressed on said device from said line, said device having an input and anoutput circuit, said input circuit comprising a control electrode, means comprising a rectifier 'connected to said output circuit for impressing astatic potential upon said control. element for controlling the transmission of current through said" device, sald static potential being dependent upon the current transmitted through said device, said potential being varied in such a manner that the gain of the device as a distortionless amplifier increases from a small value for crosstalk currents to a large value for signal currents vwithin a very small range between crosstalk .and signal voltages.

7. The. .combination with a transmission system of a line Ifor the transmission of signal voltages and means for the suppression of low level disturbing waves therein comprising a space discharge translating device having an input and an output circuit, means in said input circuit for causing the translation device to discriminate between currents of different amplitudes, said means comprising a resistance, a rectifying means for supplying a potential drop across said resistance determined by the current in said transmission line and a resistance'in the input circuit of said rectifier means for causing the current supplied to said rst mentioned resistance to increase rapidly around the capable of amplify- 

